1. Field of the Invention
The invention relates to a process for preparing aqueous polymer dispersions based on vinyl esters, ethylene and optionally further comonomers, the polymer dispersions having a solids content of greater than 65% and a viscosity less than 10,000 mPaxc2x7s, and to the use thereof.
2. Background Art
Protective-colloid-stabilized polymers are employed, particularly in the form of their aqueous dispersions or in the form of water-redispersible polymer powders, in a variety of applications. Examples include coating compositions and adhesives for a very wide variety of substrates, for example as cementitious tile adhesives, as wood glue, or as flooring adhesives. Many varieties of polyvinyl alcohols are used as protective colloids. Polyvinyl alcohols are desirable as protective colloids, because they contribute to the strength of the end product, for example improved tensile bonding strength in tile adhesives as compared to systems stabilized by low molecular weight compounds (emulsifiers). Furthermore, it is desirable from an environmental standpoint to forego the use of low molecular weight surfactants, especially alkylphenol ethoxylates, and to carry out stabilization using protective colloids exclusively.
A high solids content in such polymer dispersions is desirable for a number of reasons. High solids content affords logistical advantages, since less water has to be transported, as well as technical advantages, such as more rapid drying of the adhesives. Numerous patents disclose the preparation of high solids dispersions. The standard approach involves producing a bimodal or polymodal particle size distribution using specific polymerization techniques. The products are stabilized predominantly by emulsifiers, or by both emulsifier and polyvinyl alcohol. Emulsifier-free dispersions with a high solids content have not been available to date.
U.S. Pat. No. 5,426,146 provides a process for preparing high solids dispersions by initiating polymerization in the presence of a seed latex consisting of a mixture of two or more dispersions containing relatively small particles having a size of from 20 to 100 nm. This process yields dispersions with a specific particle size distribution, which creates better flow properties. The process in question is relatively complex, with specific conditions to be observed with regard to the bulk density, the specific volume, and the particle diameter.
EP-A 614922 (U.S. Pat. No. 5,430,092) describes a process for preparing aqueous polymer dispersions with high solids content and good flow properties in the presence of a seed latex having a particle size of from 90 nm to 500 nm. The seed latex which must be included in its entirety in the initial charge. The polymerization takes place in a multistage feed process in which a defined portion of the monomers is polymerized initially and then a specific polymer conversion must be attained. Again, the highly complex procedure is disadvantageous.
DE-A 4213968 (U.S. Pat. No. 5,340,859) describes a procedure for preparing high solids dispersions in the presence of two seed latices, of which one contains both large (greater than 300 nm) and relatively small (less than 50 nm) particles and is metered in gradually, whereas the second seed latex is included in its entirety in the initial charge. Additionally, there are certain parameters which must be observed with regard to conversion and molar number of the seed latex particles. Here again, the highly complex procedure is disadvantageous.
EP-A 814103 (U.S. Pat. No. 6,028,135) relates to a procedure for preparing aqueous polymer dispersions of high solids content that is intended to avoid the formation of gel specks, in particular those produced during polymerization. This improvement is realized by preemulsifying the majority of the polymerizable monomers into a seed latex, which is then metered into the polymerization.
DE-A 19642762 relates to a process for preparing polymer dispersions of high solids content, where, to avoid the formation of gel specks, functional comonomers are copolymerized, and during the polymerization, the pH is altered. Polymerization first takes place at pH conditions under which the functional groups are in neutralized form, the pH then being changed such that the functional groups are converted into charged groups.
DE-A 19645427 also relates to a process for preparing polymer dispersions with high solids content where polymerization is conducted in the presence of functional comonomers, but utilizes a two-stage feed technique in which, in the first stage, the feed rate is continuously increased, and in the second stage polymerization is carried out at constant feed rate. No seed latex is used in the process.
EP-A 889068 describes a process for preparing vinyl acetate-ethylene dispersions of high solids content, where some of the vinyl acetate is included in the initial charge and some is metered in, the polymerization being conducted in the presence of a mixture of polyvinyl alcohol and nonionic emulsifier.
EP-A 814096 (U.S. Pat. No. 6,028,135) describes a process for preparing dispersions having a solids content of at least 70%. In order to avoid microcoagulum, a water-miscible organic solvent is used during the emulsion polymerization. Stabilization is carried out using an emulsifier.
EP-A 785970 describes dispersions having a solids content of at least 77%. The particle size distribution is at least bimodal. Polymerization takes place in the presence of a seed latex which is introduced in the initial charge. Stabilization is again carried out with emulsifier and, if desired, also with polyvinyl alcohol.
U.S. Pat. No. 6,001,916 describes a process for preparing vinyl acetate-ethylene dispersions having a solids content of at least 65%. The polymerization is preferably carried out with the use of polyvinyl alcohol in combination with a nonionic emulsifier as dispersant. In the procedure described therein, all of the seed latex is included in the initial charge.
In U.S. Pat. No. 5,936,020 a process is described for preparing vinyl acetate-ethylene copolymer dispersions having a solids content of at least 65%. The polymerization takes place in the presence of a dispersant combination comprising polyvinyl alcohol, nonionic emulsifier, and the salt of a polyacid.
EP-A 928798 relates to the preparation of aqueous dispersions of vinyl acetate-ethylene copolymers having a solids content of at least 65%. The dispersions are stabilized by means of a specific emulsifier system comprising polyvinyl alcohol and nonionic emulsifier.
It was an object of the invention to provide a stable polymer dispersion based on vinyl esters, ethylene and, if desired, further comonomers the dispersion having a solids content of greater than 65% and a Brookfield viscosity less than 10000 mPaxc2x7s, stabilized by means of polyvinyl alcohol.
The invention provides a process for preparing copolymers based on vinyl ester and ethylene, and optionally further comonomers, in the form of aqueous dispersions having a solids content greater than 65% and a Brookfield (20 sxe2x88x921) viscosity of less than 10,000 mPaxc2x7s by means of free-radical initiated polymerization of one or more vinyl ester monomers and ethylene, and optionally further copolymerizable monomers, in the presence of one or more protective colloids, polymerization being conducted in the presence of from 2 to 15% by weight of seed latex, based on the amount of vinyl ester, by:
a) polymerizing the comonomers, without adding emulsifier, and in the presence of one or more polyvinyl alcohol protective colloids, the polyvinyl alcohols having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% by weight aqueous solution of from 3 to 30 mPaxc2x7s, and
b) introducing the seed latex in a fraction of from 30 to 75% by weight before beginning the polymerization and metering in the remainder during the polymerization, the seed latex having an average particle size (weight average Dw) of from 200 to 2000 nm.
Suitable vinyl esters are those of carboxylic acids having 1 to 12 carbon atoms. Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate, and vinyl esters of xcex1-branched monocarboxylic acids having 9 to 11 carbon atoms, examples being VeoVa9(copyright) or VeoVa10(copyright) (trade names of Shell). Vinyl acetate is particularly preferred. The vinyl esters are generally copolymerized in amounts of from 30 to 90% by weight, based on the overall weight of the monomers.
Ethylene is generally copolymerized in an amount of from 1 to 40% by weight based on the overall weight of the monomers.
Suitable further comonomers include esters of acrylic acid or methacrylic acid, vinyl halides such as vinyl chloride, and olefins such as propylene. Suitable methacrylic or acrylic esters are esters of unbranched or branched alcohols having 1 to 15 carbon atoms such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate, and norbornyl acrylate. Preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate, and 2-ethylhexyl acrylate. These comonomers may be copolymerized in amounts of from 1 to 40% by weight based on the overall weight of the monomers.
If desired it is possible to copolymerize from 0.05 to 10% by weight, based on the overall weight of the monomer mixture, of auxiliary monomers. Examples of auxiliary monomers are ethylenically unsaturated monocarboxylic and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid, and maleic acid; ethylenically unsaturated carboxamides and carbonitriles, preferably acrylamide and acrylonitrile; monoesters and diesters of fumaric and maleic acids such as the diethyl and diisopropyl esters; maleic anhydride; and ethylenically unsaturated sulfonic acids and their salts, preferably vinylsulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid. Further examples are precrosslinking comonomers such as polyethylenically unsaturated comonomers, examples being divinyl adipate, diallyl maleate, allyl methacrylate, and triallyl cyanurate, or post-crosslinking comonomers, examples being acrylamidoglycolic acid (AGA), methylacrylamidoglycolic acid methyl ester (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide (NMMA), N-methylolallylcarbamate, alkyl ethers such as the isobutoxy ether or esters of N-methylacrylamide, of N-methylolmethacrylamide and of N-methylolallylcarbamate. Also suitable are epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate. Further examples are silicon-functional comonomers, such as acryloxypropyltri(alkoxy)silanes and methacryloxypropyltri(alkoxy)silanes, vinyl trialkoxysilanes, and vinylmethyldialkoxysilanes, examples of alkoxy groups present being methoxy and ethoxy radicals and ethoxypropylene glycol ether radicals. Mention may also be made of monomers containing hydroxyl or CO groups, examples being methacrylic and acrylic acid hydroxyalkyl esters such as hydroxyethyl, hydroxypropyl and hydroxybutyl acrylates or methacrylates and compounds such as diacetoneacrylamide and acetylacetoxyethyl acrylates or methacrylates.
Preference is given to comonomer mixtures of vinyl acetate with from 1 to 40% by weight of ethylene; comonomer mixtures of vinyl acetate with from 1 to 40% by weight of ethylene and from 1 to 50% by weight of one or more further vinyl ester comonomers having 1 to 12 carbon atoms in the carboxylic acid radical such as vinyl propionate, vinyl laurate, vinyl esters of xcex1-branched carboxylic acids having 9 to 11 carbon atoms such as VeoVa9, VeoVa10, VeoVa11; mixtures of vinyl acetate, from 1 to 40% by weight of ethylene, and preferably from 1 to 60% by weight of acrylic esters of branched or unbranched alcohols having from 1 to 15 carbon atoms, especially n-butyl acrylate or 2-ethylhexyl acrylate; mixtures of from 30 to 75% by weight of vinyl acetate, from 1 to 30% by weight of vinyl laurate or vinyl esters of an xcex1-branched carboxylic acid having 9 to 11 carbon atoms, and from 1 to 30% by weight of acrylic esters of branched or unbranched alcohols having 1 to 15 carbon atoms, especially n-butyl acrylate or 2-ethylhexyl acrylate, which also contain from 1 to 40% by weight of ethylene; and mixtures of vinyl acetate, from 1 to 40% by weight of ethylene, and from 1 to 30% by weight of vinyl chloride; where such mixtures may further contain the abovementioned auxiliary monomers in the stated amounts, and the weight percentages of all monomers add up to 100% for each polymer.
The monomer selection and the selection of the weight fractions of the comonomers are made such as to give, in general, a glass transition temperature Tg of from xe2x88x9250xc2x0 C. to +50xc2x0 C. The glass transition temperature Tg of the polymers may be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg may also be calculated approximately in advance by means of the Fox equation. According to T. G. Fox, BULL. AM. PHYSICS SOC. 1,3, page 123 (1956): 1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn, where Xn is the mass fraction (% by weight/100) of monomer n, and Tgn is the glass transition temperature, in degrees kelvin, of the homopolymer of the monomer n. Tg values for homopolymers are listed in the POLYMER HANDBOOK, 2nd Edition, J. Wiley and Sons, New York (1975).
Polymer preparation takes place by the emulsion or suspension polymerization processes, preferably by emulsion polymerization, the polymerization temperature being generally from 40xc2x0 C. to 100xc2x0 C., preferably from 60xc2x0 C. to 90xc2x0 C. The pressure generally reaches 5 to 100 bar. The polymerization is initiated with water-soluble or monomer-soluble initiators or redox initiator combinations that are customary for emulsion and suspension polymerization, respectively. Examples of suitable water-soluble initiators are the sodium, potassium and ammonium salts of peroxydisulfuric acid, hydrogen peroxide, t-butyl peroxide, t-butyl hydroperoxide, potassium peroxydiphosphate, tert-butyl peroxypivalate, cumene hydroperoxide, isopropylbenzene monohydroperoxide, and azobisisobutyronitrile. The sodium, potassium and ammonium salts of peroxydisulfuric acid and hydrogen peroxide are preferred. Suitable monomer-soluble initiators are dicetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, and dibenzoyl peroxide. The initiators are generally used in an amount of from 0.01 to 0.5% by weight, based on the overall weight of the monomers.
Suitable redox initiators are combinations of the abovementioned initiators with reducing agents. Suitable reducing agents are the sulfites and bisulfites of alkali metals and ammonium, an example being sodium sulfite; the derivatives of sulfoxylic acid such as zinc or alkali metal formaldehyde-sulfoxylates, for example, sodium hydroxymethanesulfonate (Brxc3xcggolit); and (iso)ascorbic acid. Sodium hydroxymethanesulfonate and (iso)ascorbic acid are preferred. The amount of reducing agent is preferably from 0.01 to 0.5% by weight based on the overall weight of the monomers.
In order to control the molecular weight it is possible to use regulating substances (i.e., chain transfer agents) during the polymerization. Regulators are normally used in amounts of from 0.01 to 5.0% by weight based on the monomers to be polymerized, and are metered in separately or as a premix with reaction components. Examples of such substances are n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol, and acetaldehyde. Preferably, no regulating substances are used.
Suitable protective colloids are partially saponified polyvinyl alcohols having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% by weight aqueous solution of from 3 to 30 mPaxc2x7s (Hxc3x6ppler method at 20xc2x0 C., DIN 53015). Also suitable are partially saponified, hydrophobically modified polyvinyl alcohols having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% by weight aqueous solution of from 3 to 30 mPaxc2x7s. Examples of these are partially saponified copolymers of vinyl acetate with hydrophobic comonomers such as isopropenyl acetate, vinyl pivalate, vinyl ethylhexanoate, vinyl esters of saturated, xcex1-branched monocarboxylic acids having 5 or 9 to 11 carbon atoms; dialkyl maleates and fumarates such as diisopropyl maleate and diisopropyl fumarate; vinyl chloride; vinyl alkyl ethers such as vinyl butyl ether; and olefins such as ethene and decene. The fraction of hydrophobic units is preferably from 0.1 to 10% by weight based on the overall weight of the partially saponified polyvinyl alcohols. It is also possible to use mixtures of the abovementioned polyvinyl alcohols.
Further preferred polyvinyl alcohols are partially saponified, hydrophobicized polyvinyl alcohols obtained by a polymer-analogous reaction, an example being acetalization of the vinyl alcohol units with C1 to C4 aldehydes such as butyraldehyde. The fraction of the hydrophobic units is preferably from 0.1 to 10% by weight, based on the overall weight of the partially saponified polyvinyl acetate. The preferred degree of hydrolysis is from 80 to 95 mol %, more preferably from 85 to 94 mol %, the Hxc3x6ppler viscosity (DIN 53015, Hxc3x6ppler method, 4% by weight aqueous solution) is preferably from 3 to 30 mPaxc2x7s, more preferably from 2 to 25 mPaxc2x7s.
Maximum preference is given to polyvinyl alcohols having a degree of hydrolysis of from 85 to 94 mol % and a Hxc3x6ppler viscosity, in 4% by weight aqueous solution, of from 3 to 15 mPaxc2x7s (Hxc3x6ppler method at 20xc2x0 C., DIN 53015). The protective colloids are attainable by means of processes known to the skilled worker. The polyvinyl alcohols are generally used in the polymerization in amounts of from 1 to 20% by weight based on the overall weight of the monomers. In the process of the invention, polymerization is carried out without the addition of emulsifiers. The protective colloid fraction may either be included completely in the initial charge or else partly included in the initial charge and partly metered in. It is preferred to include at least 2% by weight of the protective colloid in the initial charge; most preferably, the protective colloid fraction is included completely in the initial charge.
The monomers may be included in their entirety in the initial charge, metered in in their entirety, or included partly in the initial charge, with the remainder metered in after the polymerization has been initiated. A preferred procedure is to include from 20 to 50% by weight based on the overall weight of the monomers, in the initial charge and to meter in the remainder. The feeds may be carried out separately (spatially and temporally) or some or all of the components to be metered may first be emulsified prior to metering in.
Depending on their chemical nature, the auxiliary monomers may likewise all be included in the initial charge or metered in. Partial inclusion in the initial charge and partial metering, are also possible. The auxiliary monomers are preferably metered in or included in the initial charge as a function of their copolymerization parameters. The monomer conversion is controlled with the initiator feed. The initiators are preferably metered in entirely.
Essential to the invention is the use of a seed latex. The seed latex is used in an amount of from 2 to 15% by weight (solids fraction s/s), based on the amount of vinyl ester used. Also essential to the invention is that the seed latex is not included entirely in the initial charge but instead is in part metered in. It is preferred to include from 30 to 75% by weight of the total amount of seed latex in the initial charge. The remaining 25 to 70% by weight are supplied in the course of the polymerization.
As the seed dispersion it is possible to use all customary, polyvinyl alcohol-compatible homopolymers or copolymers of ethylenically unsaturated monomers, in the form of aqueous dispersions, irrespective of the monomer composition and stabilization. Preference is given to aqueous dispersions of vinyl acetate homopolymers and copolymers, especially vinyl acetate-ethylene copolymers, that are stabilized with polyvinyl alcohol. Examples thereof are vinyl acetate polymers and vinyl acetate-ethylene copolymers with an ethylene fraction ranging from 1 to 40%, which may, if desired, also include further comonomers from the group consisting of vinyl esters, (meth)acrylic esters, vinyl chloride, and also the auxiliary monomers referred to earlier above, in each case preferably in the abovementioned amounts. The preferred particle size of the seed dispersion is from 200 to 2000 nm, with particular preference from 500 to 1500 nm, (in each case expressed as weight average particle size, Dw. The solids content is preferably from 50 to 65%, with particular preference given to the range from 55 to 60%. The viscosity of the seed dispersion is preferably less than 3000 mPaxc2x7s (Brookfield, 20 sxe2x88x921)
After the end of the polymerization, the batch may be postpolymerized using known methods in order to remove residual monomers, by means, for example, of a postpolymerization initiated with redox catalysts. Volatile residual monomers may also be removed by means of distillation, preferably under reduced pressure, and, if desired, assisted by the passage of inert entraining gases such as air, nitrogen or steam, through or over the batch.
The aqueous dispersions obtainable by the process of the invention have a solids content of greater than 60% by weight, more preferably greater than 65% by weight, and most preferably greater than 70% by weight. The viscosity of the dispersions of the invention is preferably less than 10,000 mPaxc2x7s, more preferably less than 5000 mPaxc2x7s. Preferred dispersions are those with low thixotropy, that is to say a low dependency of viscosity with shear stress.
The aqueous dispersions obtainable by the process of the invention can be used, if desired, to prepare water-redispersible polymer powders. For this purpose the aqueous dispersions, optionally following the addition of protective colloids as a spraying aid, are dried. Drying may be accomplished, for example, by fluidized bed drying, freeze drying or spray drying. Preferably, the dispersions are spray dried.
The aqueous polymer dispersions and the powders obtainable therefrom may be employed in all fields of application typical for these products. Examples include chemical products for the construction industry, for example, alone or in conjunction with hydraulically setting binders such as cements (Portland, aluminate, pozzolanic, slag, magnesia, and phosphate cements), gypsum and water glass, for the production of building adhesives, especially tile adhesives and exterior insulation adhesives; and for plasters and renders, filling compounds, trowel-applied flooring compounds, leveling compounds, grouts, jointing mortars, and paints, as well as binders for coating compositions and adhesives generally, or as coating and binding materials for textiles and paper or wood.
The examples which follow serve to illustrate the invention:
The seed dispersion used in the examples was an aqueous dispersion of a copolymer containing 85% by weight vinyl acetate and 15% by weight ethylene. The dispersion was stabilized by means of 8% by weight, based on the copolymer weight, of a polyvinyl alcohol having a degree of hydrolysis of 88 mol % and a Hxc3x6ppler viscosity of 4 mPaxc2x7s. The dispersion had a solids content of 55%, a Brookfield viscosity (20 sxe2x88x921) of 600 mPaxc2x7s, and an average particle size (weight average Dw) of 900 mn.